1. Field of the Invention
The present invention relates to an optical information recording/reproducing apparatus in which intense light is set to (held at) a level of power having an objective value before actual recording is started.
2. Description of the Related Art
Optical recording/reproducing apparatuses (hereinafter referred to as optical disk apparatuses) for recording information on or reproducing information from an optical recording medium (hereinafter referred to as an optical disk) by radiating an optical beam have recently been put into practical use.
In the above-described type of optical disk apparatus, an optical beam, emitted from a light source such as a semiconductor layer, is radiated on an optical disk having a light-sensitive thin metal film, a thermoplastic or an optical magnetic recording material provided on its surface in order to cause physical deformation of the optical disk or to change a property of the disk such as reflectivity or the direction of magnetization, by which means information is recorded at high density and the recorded information can when necessary be erased. The recorded information can be reproduced by detecting reflected or transmitted light.
As the density at which recording is effected has become higher in recent years, the recording data modulation system and the pulse recording system are leading technologies in the mainstream of optical disk apparatus development, and various methods of stabilizing the recording light power of a laser serving as a light source have been proposed, most of them involving A.P.C. (Automatic Power Control) in which the intensity of light emitted is monitored and controlled utilizing a feed-back loop when any recording light is to be emitted. However, this A.P.C. System has a problem in that highly accurate control is impossible in some cases, depending on the recording data modulation system employed. For the M.sup.2 FM modulation system, each data signal has, for example, a constant duty cycle, and A.P.C. is therefore attained by feeding back the average value of the monitor output of the light power. For the 2-7 modulation system, the duty cycle of each data signal is not constant, remarkably fluctuating the average value of the monitor output. In consequence, feed-back of the average value does not ensure a stable setting of the light power of a laser to an objective value.
This light power adjusting technique which utilizes feed-back is useful when an optical beam of constant intensity is emitted from a laser light source for reproduction or erasure. However, when information is recorded, a laser beam is modulated in response to the information to be recorded. As a result, where a signal to be fed back and controlled (hereinafter referred to as a feed-back signal) is obtained utilizing an average value detection, the output of a feed-back signal fluctuates in accordance with the average value for a modulated area in modulation methods other than the M.sup.2 FM and EFM methods, and stable light power control is therefore impossible if the fluctuation is intense. No problem arises with the M.sup.2 FM and EFM modulation methods because the average value for a certain area is substantially constant.
Accordingly, it has been proposed to peak hold the intensity of pulsed light emitted from a laser when a data is recorded and to perform feed-back on based this value so as to obtain an objective light power. However, as the speed at which data is recorded becomes higher, the interval during which a diode laser emits recording light becomes shorter, one of the shortest intervals at present being 100 nsec or less. This makes timing of peak holding or holding of an accurately calculated peak value for a very short period of time technically difficult.
More specifically, where a feed-back signal is obtained by a peak hold detection, each pulse has a width of about 100 nsec in the pulse recording of RZ and RB. It is therefore difficult to provide a circuit having a time constant which is capable of holding such a pulse signal, and it is impossible to obtain a stable feed-back signal.